Seesion1 summary Unwanted Beam Workshop (UBW 2012) Dark Current Issues for Energy Recovery Linacs

2 BERLinPro: Berlin Energy Recovery Project funding decision late 2010, start of project 2011 extensive gun development: Gun0.1 (2011), Gun0.2 (2012) and Gunlab 2015: first electrons from Gun2 & Booster  6 MeV 2017: merger 2018: 50 MeV recirculation

MEASURES AT DESIGN STAGE TO MINIMIZE EFFECT OF DARK BERLINPRO 3 Do not plan to operate at ultimate gradients Collimation is an issue Protect potentially weak places (water cooling of vacuum chambers) Diagnostics to control the unwanted beam losses Do the modeling … but do not believe it…

JLab IR/UV Upgrade E beam 135 MeV Bunch charge:60 pC – UV FEL 135 pC – IR FEL Rep. rate up to MHz 25 μJ/pulse in 250–700 nm UV-VIS 120 μJ/pulse in 1-10 μm IR

Conclusion / Summary  JLab FEL (IR/UV Upgrade)  RF gradients in LINAC always require attention, set radiation background level (FE)  HV-DC gun very tricky to process (new gun should improve it a lot) “Catch-22”  Drive Laser transport if made very carefully, seems to be not a problem  Drive Laser rep. rate control (EO cells) always need attention (extinction ration drifts)  Cathode suffers when conditioning and from breakdowns, still makes beam as needed, but scatters DL light – generates some halo  Non-linear beam dynamics is responsible for some fraction of the halo. When setting up for high current operation, a lot of effort and time goes in to “fitting” the halo through the recirculator, such that peak beam brightness does not suffer.  Radiation monitors, BLMs and vacuum are used as tuning diagnostics  CEBAF  NP detectors (background) require essentially no beam halo  Large statistics of cavity performance and its evolution (FE)  Direct effects of FE – RF trip rate, reduction of max. possible energy  Vacuum events related to beam loss (both high and very low current)

Lasing (1) Lasing (4) Lasing (2) groups of undulators and IR FELs (under construction and fabrication) One track in vertical plane (terahertz FEL) Common for all FELs accelerator system 6 Two tracks in a horizontal plane (IR FEL) Four tracks in a horizontal plane Full scale Novosibirsk FEL 6

Conclusion Unwanted beam losses can exist due to low-energy effects, lasing, and insufficiently flexibility of system in multipass mode. Protection system is working well in regular mode, should be changed for tuning and adjusting mode. Present status: at this time, in Novosibirsk FEL based on one-turn ERL operates with the highest power in terahertz region; FEL, based on the first in the world two-passes ERL, operates in infrared region; 95% energy recovery efficiency and 1 mA current are achieved at the four- pass ERL. 7